QuickSynergy simplifies the Synergy interface. Synergy takes a little while to figure out because all of the connections between computers are made from the command line in a somewhat cryptic fashion. QuickSynergy is a GUI, thereby simplifying the process of connecting your computers.
To install QuickSynergy, just go to a command prompt and type:
sudo apt-get install quicksynergy
I recommend using a Linux machine as the host, and setting up Windows and OSX machines as clients. Setting up the host is the hard part and it is easiest to do in Linux. For more info, visit the QuickSynergy project page.